This invention relates to electronic hybrid circuits used for 2-wire to 4-wire communication and particularly to an active hybrid circuit equipped with high impedance current sources and suitable for use in a telephone system for communication between a 2-wire bidirectional central office switching path and a distantly located operator console via a long haul 4-wire trunk.
Telephone companies frequently locate operator consoles at great distances from the central switching machine which they serve. The location is sometimes determined by the practical availability of operators, for example, only in surburban areas displaced from an urban switching office. In such a situation, it is a common practice for the telephone company to utilize a 4-wire long haul trunk arrangement from the remote operator location to the distant switching office and a hybrid circuit for interface communication between the 4-wire trunk and the conventional 2-wire switching network in the office.
A problem in such prior art long haul trunk arrangements is that a large number of circuit components have heretofore been required for furnishing customers with high quality transmission and reducing the impedance loading effect of the hybrid circuit and the 4-wire trunk on the 2-wire central office path. The components usually include a sophisticated negative impedance converter and balancing network. Despite sustained efforts to reduce the cost and size of such arrangements as well as other innovations in circuit design and fabrication techniques, negative impedance facilities are yet needed and prove to be costly and complex adjuncts to the hybrid circuitry. Another disadvantage is that the negative impedance balancing networks occasionally require time consuming and critical empirical adjustments to cancel 4-wire trunk capacitances and other imbalances.